Infectious diseases kill more people than any other single cause. Genome-Wide-Association-Studies in humans have demonstrated that genetic variations in genes involved in host defense can affect individuals' susceptibility to infection. I most cases, such reverse genetic approaches to understanding genetic contributions to disease are dependent on animal models that allow analysis of the effects of genetic variance in the absence of other variables. The lack of such models and the knowledge they yield poses a significant barrier for the future development of personalized treatment and health care for individuals carrying these genetic variations. Here we propose the construction of mouse gene knockin models to study the function of two variants of the human gene that encodes STING, a type I IFN stimulator that in mice is essential for defense against viral infections. We recently reported that there are two STING variants in man, each encoding three amino-acid changes (R71H-G230A-R293Q). We refer to these as WT and HAQ. Our in vitro studies have demonstrated that relative to WT the HAQ-STING variant has lost >90% of the ability to stimulate type I IFN production upon pathogen infection. Furthermore, the HAQ-STING variant has a dominant negative functional effect in vitro. Surprisingly, ~20% of Americans carry at least one copy of the HAQ-sting variant. It is our long-term goal to understand the molecular mechanisms and in vivo function of this frequent, potentially nonfunctional, human STING variant that may affect disease susceptibility of millions of Americans. To achieve this goal, we propose to develop two mouse models for the human STING variants. The two models will include 1) a minimalist knockin of SNPs into the mouse gene and 2) a knockin of SNPs plus unique human contextual sequence of potential importance in function. It is noteworthy that mouse and human STING proteins share 83% amino acid sequence homology. Thus in the first model (mHAQ mouse), we will make a STING knock-in in which the mouse equivalent amino acids will be changed to those in human HAQ-STING (Aim 1). In the second model we will accommodate the limited regions of non-homology, yet take advantage of the fact that mouse and human STING genes have the same exon - intron structure. We will make a STING knock-in that expresses a chimeric form of human-mouse STING, in which the entire N-terminal 172aa is replaced by the N-terminal of 173aa of human STING (cHAQ mouse) (Aim 2). This mouse will also contain G230A and R293Q. Appropriate WT mice will also be constructed. As time permits we will use these models to study the effects of HAQ on resistance to infection and on STING signaling function.